A can lid of this type generally consists of a rivet and dimples which are gibbously and upwardly shaped, an opening which is demarcated by a main score and arranged across the rivet for the dimples, and a tab which is fixed to the rivet such that one end part thereof overlaps the opening and the other end thereof overlaps the dimples on an upper surface of the can lid main body. On the other end of the tab, a finger hole is perforated in a thickness direction.
In general, two dimples are formed on the upper surface of the can lid main body, and each of these dimples is located so as to face each other across a straight line which connects the center of the opening or the tab in the width direction with the center line of the can lid main body. These dimples support the lower surface of the tab, such that the tab is prevented from rotating, even when a force which rotates the tab around the rivet acts thereon.
The can lid thus constituted is wound and fastened to a can body so as to form a can which is filled with a content. The can is constructed such that when an outwall part (referred to as “a pull-up part”, hereinafter), which is located on the other end of the tab, of the pull-up part which forms a finger hole of the tab, is pulled up by a consumer, then one end part of the tab pushes the opening and breaks a main score thereby perforating the opening. Here, a part of the main score is located on a coining part which is constituted to be harder than the other part, which is a periphery of the rivet, which is contiguous to the rivet on the upper surface of the can lid main body, and constructed such that the main score can easily be broken when pulling up the pull-up part of the tab.
Incidentally, hitherto there has been a problem in that a finger hardly enters into a gap between the upper surface of the can lid main body and the pull-up part of the tab and hence it is difficult to pull up the pull-up part successfully. Hitherto, as is disclosed in the following documents Japanese Examined Utility Model Application No. 7-44595; Japanese Unexamined Patent Application, First Publication No. 4-44950; Japanese Utility Model (Granted) Publication No.2508637; Japanese Patent (Granted) Publication No. 3,468,548, various means for solving this problem have been proposed.
At first, Japanese Examined Utility Model Application No. 7-44595 1 has disclosed an invention which consists of a recess for inserting one's finger and extending it into the radial direction of the can lid main body from a ring-like groove which constitutes the outer periphery of the upper surface of the can lid main body, underneath the pull-up part of the tab.
Although, it becomes easy to insert one's finger into the gap between the upper surface of the can lid inserting one's finger and the pull-up part in accordance with the invention, the invention is not applicable to the case in which pressure in the can is high this is because the compressive strength of the can lid may deteriorate since the recess for inserting one's finger is extended from the ring-like groove to the right under the pull-up part. Moreover, plural can lids are stacked and transferred through a tube in the production process of the can lid. When transferring the can lids, a lower surface of a recess for inserting one's finger comes into contact with an upper surface of a tab of the other can lid which is located facing the lower surface so as to form a space between curling parts of the can lids which are adjacent to each other in a stacking direction, i.e. an accordion-like situation, and as a result, stacking performance deteriorates and it becomes difficult to provide favorable transferring performance.
In addition, there has been a case that liquid paraffin which has been adhered to the tab of another can lid is transferred to the lower surface of the recess for inserting one's finger in the one can lid which is located facing the tab. There has been a problem in that if the content is poured from the can in which one can lid has been wound and fastened into the other vessel, then it becomes difficult that the content sparkle in the case in which the content is for example beer.
Next, Japanese Unexamined Patent Application, First Publication No. 4-44950 disclosed a can lid which is equipped with a two-stage shape such that the lower surface of one end comes into contact with the upper surface of the can lid main body, and simultaneously the lower surface of another end departs upwardly from the upper surface of the can lid main body.
Since a gap has been formed between the lower surface of another end of the tab having a pull-up part and the upper surface of the can lid main body in the present invention, it is possible to insert one's finger easily into the gap, thereby realizing a favorable opening property.
However, there is a problem in that it is difficult to provide favorable transferring performance and foaming performance, similar to the can lid disclosed in Patent document 1, because the upper surface of the tab of one can lid comes into contact with the lower surface of another can lid which is placed facing the can lid, when transferring the can lid through a tube in the production process of a can lid.
Next, Japanese Utility Model (Granted) Publication No.2508637 disclosed an invention in which a secondary-score is formed at a location which is opposed to the pull-up part of the tab in the upper surface of the can lid main body.
In accordance with the invention, it is possible to prevent the gap between the pull-up part and the upper surface of the can lid from decreasing when the inner pressure of the can increases, because the area extending from the opening part over the secondary-score is expanded and deformed when the inner pressure of the can increases, instead of expanding and deforming the opening part larger than the other part in the upper surface of the can lid main body.
However, the invention merely expands and deforms all of the area from the opening part over the secondary-score, and does not increase the gap between the upper surface of the can lid main body and the pull-up part of the tab. In addition, Japanese Examined Utility Model Application No. 7-44595 has disclosed an invention in which a bead is formed at a location opposed to the pull-up part of the tab in the upper surface of the can lid main body. However, in such a constitution, there is a possibility that the tab and the bead come into contact with each other due to the vibration which acts on the can when transferring the can, thereby breaking the main score which demarcates the opening part.
In order to solve the above problem concerning the opening performance of a can lid, the inventors of the present invention thoroughly conducted research and have found the reason why the distance between the lower surface of the pull-up part and the upper surface of the can lid main body decreases. When an opening part is formed on the upper surface of the can lid main body, in particular when a part of the main score is formed to the coining part, a metal on the upper surface of the can lid main body which is located between part of the main score and the rivet flows toward a rivet side, thereby slanting the rivet to another part side of the tab (side opposed to the opening part across the rivet), and simultaneously, the one end part of the tab which is fixed to the rivet moves upward, whereas another end moves downward and slants so as to decrease the distance between the lower surface of the pull-up part and the upper surface of the can lid main body. That is, they have formulated a hypothesis that the distance between the pull-up part of the tab and the upper surface of the can lid main body had already decreased in the simplicial can lid before being wound and fastened to the opening of a can body, and hence it was as difficult as ever to insert one's finger into the gap between the upper surface of the can lid main body and the pull-up part of the tab before or after an inner pressure is applied. It should be noted that it was confirmed that the angle of the slanting should not be less than 4° from the center line which is across rectangularly the upper surface of the con lid main body, as a simplicial can lid.
Under such an opinion, as a means to prevent the slanting of the rivet, it can be proposed to provide a constitution of forming a score or a coining part for preventing the slanting of rivet onto a portion which is opposed to the opening part across the rivet in the upper surface of the can lid main body. A can lid which resembles to such a constitution has been disclosed in, for example, Japanese Patent (Granted) Publication No. 3,468,548; Japanese Unexamined Patent Application, First Publication No. 8-164935; Japanese Unexamined Patent Application, First Publication No. No. 8-11882.
First, Japanese Patent (Granted) Publication No. 3,468,548 has disclosed a can lid, which is equipped with a secondary score which radially extends from the center part of the upper surface of the can lid main body to the outer periphery at a part which is opposed to the opening part across the rivet in the upper surface of the can lid main body, in order to provide a can lid whereby the contents are easily consumed. The part which is surrounded by the secondary-score is located of a position which faces to the lower surface of the one end part of the tab when the tab is rotated in a plane which is horizontal to the upper surface of the can lid main body, and as a result, when the other end part is pulled up after rotating the tab as mentioned above, then the part surrounded by the secondary-score is pushed by one end so as to be opened, thereby accommodating one end inside the can through the opening part.
However, the can lid has a problem in that the compressive strength and the impact resistance may deteriorate, because part of the secondary-score must be located at a coining part in order to open the part surrounded by the secondary-score.
Next, Japanese Unexamined Patent Application, First Publication No. 8-164935 has disclosed a can lid in which a secondary score is disposed as an easily-bent portion on an opposite location to an opening part across a rivet in the vicinity of a rivet where stress is likely to be concentrated when a content filled can falls. This can lid can prevent stress from being concentrated at the main score by dispersing stress which acts the vicinity of the rivet due to impact upon being dropped or water impact action, thereby preventing the main score from bending deformed and generating cracks.
However, the secondary-score disclosed in Japanese Unexamined Patent Application, First Publication No. 8-164935 is formed in the vicinity of a rivet which is adjacent to the rivet in the upper surface of the can lid main body, and hence the secondary-score may be located in the coining part, thereby it is not possible to securely prevent the deterioration in compressive strength and impact strength of a can lid due to the provision of the secondary-score.
Moreover, Japanese Unexamined Patent Application, First Publication No. No. 8-11882 has disclosed a can lid which is equipped with plural secondary scores in the vicinity of a rivet, each of which has been formed to be linear so as to surround the rivet, thereby forming an easily-bent and deformed part in the vicinity of the rivet so as to reduce an initial pulling force for opening an opening part by pulling a pull-up part up.
However, these secondary scores may be located on a coining part, as shown in FIG. 2 of Japanese Unexamined Patent Application, First Publication No. No. 8-11882, and hence it is not possible to securely prevent the deterioration in compressive strength and impact strength of a can lid, similar to Japanese Unexamined Patent Application, First Publication No. 8-164935 above.